Checks Written on Closed Account?

Hello, I have a question in regard to my fiance. We at one time had joint separate accounts through Wachovia. We wrote checks to Herff Jones for cap and gowns for graduation. Well we had written them out of the incorrect account, because we decided to close her separate account and consolidate into one account. Well the check bounced due to it being a closed account. Now a collection agency is trying to hit me up for enormous fees. I have no problem paying what I owe, and possibly a little extra since I know its a inconvenience and it still is my fault, even though its a big mistake. But I am curious do they even have legal grounds to try and collect this debt when the checks were written when we both were 17? There are no co-signers on the account we are the only ones responsible. But I am just curious legally speaking if they have grounds to collect. Like I said I don't want you to get the wrong impression I'm not trying to weasel out of the debt, after all I do need my cap and gown, but I am trying to fight this armed robbery in fees.